1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including a zoom lens. The zoom lens may be particularly suitable for use in an image pickup apparatus, such as a video camera, a digital still camera, a broadcasting camera, and a monitoring camera, or a camera using a silver-halide film.
2. Description of the Related Art
Deterioration of optical characteristics due to diffraction in a photographic optical system having high resolution is non-negligible. Deterioration of image formation performance due to diffraction is determined by an F-number (Fno) of the photographic optical system. For this reason, the photographic optical system is required to have a small full-aperture Fno. To reduce noise at the time of photography in a dark place, the photographic optical system is required to maintain a small Fno over the entire zoom range.
In a photographic optical system with a small Fno, it is important, in consideration of chromatic aberration, to reduce chromatic spherical aberration equivalent to settlement condition (the degree of condensing) of light at a focus position over the entire visible light region. In addition, it is important to appropriately control axial chromatic aberration, which is equivalent to controlling a difference in focus position at each wavelength.
To maintain a small Fno while realizing a high zoom ratio, the total lens length becomes long, and spherical aberration, axial chromatic aberration, and a change in chromatic spherical aberration upon zooming become large. As a result, it becomes difficult to obtain high optical performance over the entire zoom range. Therefore, it becomes important to optimize the material characteristics (an Abbe number and relative partial dispersion) of lens units constituting a zoom lens.
In a positive lead type zoom lens in which a lens unit having positive refractive power is arranged closest to the object side, it is relatively easy to realize a high zoom ratio, and a change in Fno upon zooming is small.
There is known a zoom lens including, in order from an object side to an image side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power, with an Fno of about 2 or less at a wide-angle end and 3 or less even in the entire zoom range.
Japanese Patent Application Laid-Open No. 2001-194586 discusses a zoom lens with a zoom ratio of 3 and an Fno of 2.06 to 2.74.
Japanese Patent Application Laid-Open No. 2011-2503 discusses a zoom lens with a zoom ratio of about 4, an Fno at a wide-angle end of 1.85, and an Fno at a telephoto end of 2.53 to 2.76.
There is also known a zoom lens including, in order from an object side to an image side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, a fourth lens unit having negative refractive power, and a fifth lens unit having positive refractive power (Japanese Patent Application Laid-Open No. 2010-32700).
In a positive lead type zoom lens including four or five lens units, to obtain high optical performance over the entire zoom range, it is important to appropriately set refractive power, a lens configuration, and other parameters of the third lens unit. In addition, when using an aspheric surface, it becomes important to appropriately set a lens unit having an aspheric surface, the shape of an aspheric surface, and the like.